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Complete the wildlife rescuer's log entry about a mischievous raccoon.
The kitchen was a total disaster area this morning. It _________________________ through the open skylight _________________________ the raccoon entered the house, because all the doors were securely locked from the inside.
The correct answers are must have been and that.
In an it-cleft sentence, we use the relative pronoun that (not where or which) to connect the emphasized phrase to the rest of the sentence, even when emphasizing a prepositional phrase of place like "through the open skylight."
Additionally, must have been is the correct modal perfect form to express a strong logical deduction about a past event.